Engine >> Carter
The Carter BBD carburetor
The Jeep 258 (4.2L) I6 isn't a power house, but it has gobs of low
RPM torque that makes it a great engine off-road. The most common
problem I see with the engine is it's inability to idle. Between the
258 in my '81 CJ-7 and a couple friends that have 258 equipped
Wranglers, I have fixed this problem half a dozen times and the cause
of the problem has always been the same. The problem has always been
caused by clogged idle tubes that cause fuel to drip out of the venturis and make the Jeep run rich at idle. Fixing the problem is
relatively easy and once you get past this problem, you will get much
more enjoyment from the 258 and the Carter BBD carburetor.
The Carter BBD is a two barrel carburetor that was available on
late 70s to late 80s Jeeps with the 4.2L engine. There are two
variations of the carburetor, one is computer controlled and has a
stepper motor on the back side. The computer controlled version was
used after 1981. Other than that, the two versions are the same
although it seems the stepper motor version is a bit more troublesome.
The symptoms of the problem include stumbling and sputtering at idle.
In advanced cases, the Jeep will stall at every stop sign and will
only run at high RPMs. Gas mileage will suffer since fuel will just be
sloshing out at idle. Sometimes the idle will be turned up to a high
RPM to avoid the problem. Typically, the Jeep will run fine at higher
RPMs (unless there are also other problems.)
For a sure diagnosis, park the Jeep with the engine off and remove
the air cleaner cover. There should be a plate over the throat of the
carb, the choke plate. If you open the choke plate you should be able
to see down the throat of the carb and you should see two screws with
holes in the middle of them. Next to them are two passages with a
nozzle in the middle of each. This thing is known as the venturi, when
air passes by, fuel is supposed to be drawn out through the nozzles.
If the idle tubes are clogged fuel will drip from those nozzles during
In order to see if fuel drips from the nozzles at idle, you must start
your Jeep with the air cleaner cover off and look down the throat of
the carb. The Jeep Technical Service Manual recommends that, when you
do this, you cover the air cleaner with a piece of plexiglass since
the engine can backfire through the carb and a flame can shoot out.
Since I am reckless and like living dangerously, I never do this.
After you have chosen the wise or foolish path, start your Jeep and
open the choke plate. If your idle tubes are badly clogged, you will
see fuel dripping from the nozzles at idle (if your Jeep will idle at
all.) If you don't see fuel dripping, but your idle is still poor,
open the throttle a bit with your hand or have a friend hit the
accelerator. You should see two even streams of fuel and no dripping
from the nozzles. Any dripping means clogged idle tubes.
While you are doing this, make sure you don't put your hand or
anything else into rotating parts like the fan. Keep your tie away
from that thing (some people just want to look good all the time.)
Same goes if you are a "long hair, freaky" Tesla type. All joking
aside, I've heard some bad stories.
Neither removal of the carburetor nor a complete rebuild is necessary
to fix the problem. To fix the problem, start with you Jeep off and
- Remove the air cleaner cover and air cleaner. You may need to
remove a few hoses and wires to get the air cleaner out of the way.
Make sure you tag them all so you can put them back in the right
spot. It's often easiest not to remove the heater hose that goes to
the exhaust manifold, if you have one.
- Remove hoop that holds air cleaner.
- Remove the two screws holding the choke plate with a 3/16"
socket or small flat blade screw driver depending on what screws you
have holding it. Be careful not to drop the screws down the manifold
unless you enjoy fishing. *Remove the choke plate.
- If your carb has one, remove the plate on the side that covers
the choke linkage so you can access the screw holding the choke rod.
It may be necessary to drill out a rivet to get it free.
- Remove the little snap ring and screw (1/4" socket) that holds
the rod that holds the choke plate and remove the rod. A screw
driver will normally push off the snap ring.
- Remove the two screws with holes in the middle of them and
carefully remove the venturi cluster with the two little gaskets.
There should be two idle pickup tubes pushed into the venturi
sticking out of the bottom. If they have fallen out, that could
cause your idle problem.
- Blast the venturi and inside of carb with lots of carb cleaner.
Be sure not to dislodge the check ball in the center.
- With a long thin pin or piece of piano wire, make sure the idle
tubes and nozzles are clear. After you have run them through, spray
with more carb cleaner.
- Put the cleaned out venturi cluster back with the two gaskets
and two screws.
- Replace the choke rod with snap ring and screw and replace the
choke plate. *Make sure all the screws are tight that hold together
the carb body at this time. Often the screws will loosen up and dirt
will be sucked into the carb.
- Replace the plate covering the choke linkage with a sheet metal
screw, or leave it off.
- Replace the air cleaner.
Smooth idle should have returned after this simple process. If you
still have idle problems and don't have any dripping, make sure all
the vacuum hoses are hooked up and in good shape. Also check for leaks
around the intake manifold or a loose carburetor. To avoid repeated
clogged idle tubes, drill out the idle tubes to 0.032". This will
greatly reduce the frequency of clogging and it is a procedure that
was actually recommended by Jeep for a while.
An alternative to removing the choke plate and all is to just
remove the top of the carburetor. The advantage of this approach is
you can make sure there isn't a lot of junk in the bowls, you can
check your float adjustment, etc. It also may help if you have power
brakes because the master cylinder may be in the way of getting the
choke rod out.
Thanks to Randy Peterson for posting this solution a few years back.
I've used it on various Jeeps many times since.